The invention relates to an arrangement in connection with the curtain coating of a fibrous web. The invention particularly relates to an arrangement in connection with curtain coating which comprises a movable nozzle beam for providing a coating material curtain for coating a fibrous web, means for moving the nozzle beam, a support member(s) of the fibrous web for guiding the fibrous web below the coating material curtain, a return reservoir and a collecting surface connected to it for at least collecting the coating material before and/or after the coating.
In this specification, coating refers to coating but also to the application of glue i.e., sizing agent, barrier material or equivalent.
Utility model specification DE20201210493U1 describes a device for receiving coating material of a fibrous web before the start of the coating process by a fibrous web coating device into a receiver of coating material, such as a return reservoir, which device includes a receiver part which receives coating material and a cleaning device arranged into connection with the receiver which is arranged with the coating device on the same side of the fibrous web.
As known of prior art, curtain coating most commonly utilizes two different ways to start the coating or a combination of these ways. One of the known ways implements the start of coating by moving a nozzle beam; the other one uses a movable plate under the curtain; and in the combination, at the start both the nozzle beam is moved and the movable plate is used. These known ways have their own special characteristics and problems particularly related to usability and start control.
Starting by moving the nozzle beam is a technically simple arrangement as such and, at the start, coating material, such as e.g. coating color, sizing agent, barrier material or equivalent, is collected into a return reservoir and onto a collecting surface fixedly connected to it, whereby the problem can be the cleansing of the edge of the collecting surface after the start, because coating material sticks to its tip. When a coating material curtain passes the tip of the collecting surface of the reservoir, it simultaneously dirties its tip also on the bottom side i.e. on the side of the fibrous web, whereby the dirtying becomes a cleansing problem. The tip of the collecting surface cannot be cleaned during run without the danger of disturbing the stability of the curtain, because an operator or some other person doing the cleaning causes air flows in the vicinity of the curtain when cleaning, which air flows make the curtain to oscillate and thus the stability of the curtain is disturbed. Furthermore, a cleaning operation done during run is problematic for work safety, because the person doing the cleaning gets close to moving parts and the moving fibrous web. It is known that the maximum moving speed of the nozzle beam is about 100 mm/s in order to keep the stability of the curtain. It is still typical to use lower speeds, e.g. 50 mm/s.
Starting by means of a movable plate is a solution as such for keeping the tip clean but, to work, it requires a relatively quick movement, typically over 300 mm/s, in order for the coating curtain not to follow the start plate in an undesired amount, which causes harmful air flows in the area of the curtain and the stability of the curtain is disturbed.
It should be noted that, in order for the curtain coating to be successful, the stability of the curtain is of utmost importance.
One has also tried to solve the cleansing problem by moving/turning the return reservoir into a cleaning position after the start of coating but, typically, this cannot be implemented due to limited space available on the curtain coater and, on the other hand, the moving/turning of a large subassembly generates harmful air flows, whereby the stability of the curtain is disturbed.